Drilling Riser VIV Tests With Prototype Reynolds Numbers

Abstract

For deep-water riser systems, Vortex Induced Vibrations (VIV) may cause significant fatigue damage. It appears that the knowledge gap of this phenomenon is considerable and this has caused a high level of research activity over the last decades. Small scale model tests are often used to investigate VIV behaviour. However, one substantial uncertainty in applying such results is scaling effects, i.e. differences in VIV response in full scale flow and small scale flow. To (partly) overcome this obstacle, a new innovative VIV test rig was designed and built at MARINTEK to test a rigid full scale riser model. The rigid riser model is mounted vertically and can either be elastically mounted or be given a forced motion. In the present version, the cylinder can only move in the cross-flow (CF) direction and is restricted in the in-line (IL) direction. The paper reports results from a drilling riser VIV experiment where the new rest rig has been used. The overall objective of the work is to study possible VIV suppression to improve operability of retrievable riser systems with auxiliary lines by adding riser fins. These fins are normally used as devices for protection of the auxiliary lines. The test program has recently been completed and analysis is an on-going activity. However, some results can be reported at this stage and more results are planned to be published. A bare riser model was used in a Reynolds number (Rn) scaling effect study. The riser model was elastically mounted and towed over a reduced velocity range around 4 – 10 in two different Rn ranges, 75 000 – 192 000 (subcritical regime) and 347 000 – 553 000 (critical regime). The difference in the displacement amplitude to diameter ratio, A/D, is found to be significant. The elastically mounted riser was also towed with various drilling riser configurations in order to study VIV/galloping responses. One configuration included a slick joint riser model with 6 kill & choke lines; another has added riser fins too. The riser model is based on a specific drilling riser and the kill and choke lines have various diameters and have a non-symmetrical layout. The various riser configurations have also been used in forced motion tests where the towed model has been given a sinusoidal CF motion. Forces have been measured. Determination of the force coefficients is still in progress and is planned to be reported later. Scaling effects appear to be a significant uncertainty and further research on the subject is recommended. The slick joint drilling riser configuration generally increased the displacements compared to displacements of the bare riser model. The drilling riser configuration with protection fins, kill and choke lines generally reduced the displacements compared to displacements of the bare riser model. For both riser systems, tests showed that the response is sensitive to the heading of the current.

Prototype Reynolds Number Vortex-Induced Vibration Tests on a Full-Scale Rigid Riser

“…During tests, a full scale VIV test rig was hinged to the carriage. The test rig was manufactured by MARINTEK to test prototype riser models, and first used by [8]. A principle sketch of the test rig is shown in Figure 1.…”

“…In [8], one spring setup was used for all the free oscillation tests, so the system has the same natural oscillation frequency in water. To achieve different reduced velocities, the towing speed was varied, and so the Re.…”

“…The key properties of the riser models are shown in Table 1. [8], the forces acting on the test riser model were measured by use of an assembled set of one-component force transducers, which were mounted on the riser model end.…”

“…Free oscillation tests were performed on both smooth and rough riser models, at selected critical Re and a range of reduced velocities. In addition, some tests in [8] were repeated with varying Re. The test program of stationary and free oscillation tests is shown in Table 2.…”

“…To fill the knowledge gap of VIV at prototype Re and overcome shortage of applying results at lower Re to prototype Re, a new innovative VIV test rig was designed and built at MARINTEK to test a full-scale rigid riser model. This test rig was first used to study possible VIV suppression to improve operability of retrievable riser systems with auxiliary lines by adding riser fins, the model tests were carried out during 2011/2012 [8]. In MARINTEK's very first prototype Re VIV tests on a full scale riser section [8], distinct difference of response amplitude ratios were observed between 'critical' and 'sub-critical' Re flow regimes, the largest amplitude ratio in 'critical' Re is around 60% of that in 'sub-critical' Re.…”